1. Field of the Invention
This invention relates to apparatus for detecting an in-focus condition applicable to photographic instruments such as cameras, and more particularly to optical systems for focusing the reflected light from an object on photosensitive elements.
2. Description of the Prior Art
From the past, detection of the in-focus condition by sensing the degree of sharpness of an image of an object near the focal plane of an objective lens, or the so-called unsharp image dependent in-focus detecting method has been known.
FIGS. 1 to 3 are schematic views of an optical system employing the conventional typical unsharp image dependent in-focus detecting method.
FIG. 1(a) shows the system when an objective lens is in sharp focus, FIG. 2(a) shows the same when the objective lens is in near focus, and FIG. 3(a) shows the same when the objective lens is in far focus. FIGS. 1(b), 2(b) and 3(b) are waveforms of the output signals from photosensitive elements 3 and 4 in the respective operative positions.
The in-focus detecting method by the unsharp image is that the photosensitive elements 3 and 4 are provided near a prescribed focal plane 2.sub.1 of the objective lens 1 at respective positions 2.sub.2 and 2.sub.3 of forwardly and rearwardly equal axial distances from the plane 2.sub.1, and the output signals from the photosensitive elements 3 and 4 are compared with each other to detect focusing conditions of the objective lens 1.
As illustrated in FIG. 1(a), when the objective lens is in an in-focus condition, the output signals from the photosensitive elements 3 and 4, for example, with a point image, represent the same sizes of circles of diffusion and, therefore, are equal to each other in the width of waveform, D, as illustrated in FIG. 1(b). Accordingly, by sensing and comparing the widths, D, it is made possible to detect when the objective lens 1 comes to the in-focus condition.
When the objective lens 1 is in the near focus, as illustrated in FIG. 2(b), the width D of the output signal from the photosensitive element 3 becomes larger than that of the output signal from the photosensitive element 4. When in the far focus, it is inverted as illustrated in FIG. 3(b).
The thus-obtained focusing conditions of the objective lens 1 can be displayed in the field of view of the finder or fed back to adjust the objective lens by means known to those skilled in the art.
The unsharp image dependent in-focus detecting method makes sure the focusing operation is performed either visually or automatically only when the unsharpness of the image by the objective lens 1 is below a certain degree. Or otherwise the difference between the output signals of the two photosensitive elements, or the difference in contrast between the two images is left constant despite further excursion of the objective lens 1. As a result, it becomes impossible to detect the in-focus condition.
The contrast of the image on the photosensitive element surface is otherwise dependent upon the F-number (relative aperture) of the objective lens too. When the degree of unsharpness of the image becomes very large, however, any increase of the F-number of the objective lens does not result in creating a difference between the contrast signals from the two photosensitive elements, thus rendering it impossible to carry out the in-focus detection.
In such case, for the operator intends to adjust the objective lens to the in-focus condition by the signals obtained from the photosensitive elements, it becomes impossible to detect which direction the objective lens is to be moved to.
For this reason, upon preliminary movement of the objective lens in one direction, if the operator fails in finding out the in-focus condition, the objective lens is then moved in the opposite direction. Such procedure takes a long time to adjust the objective lens.